It is only in relatively recent years that society has appreciated the impact and consequences of the fact that we live in a closed ecological system. With an increase in human population and, perhaps more importantly, an increase in industrial activity the effects of ecological changes have become more apparent. One area which has received a great deal of attention is that of water quality, which may be the result of the belated recognition that not only is water of a suitable quality for human consumption a limited resource, but that good water quality is an important, if not critical, factor in the ecological chain. Consequently attention has turned not only to purification of water in local water supplies, but also to limiting the discharge of materials into streams and aquifers generally.
The classes of noxious materials (pollutants) in aqueous discharges vary over an enormously broad spectrum. Among the inorganic pollutants those toxic to a broad spectrum of biological species are especially dangerous. Although heavy metals such as lead, cadmium, and arsenic often are the first culprits thought of, inorganic water soluble cyanide is in a comparably dangerous class because of the generally low tolerance of life forms to cyanide.
The sources of cyanide are many and varied and include iron and steel manufacturing, petroleum and coal pyrolysis processes, the photographic, chemicals, and pharmaceutical industries, precious metal mining and metal finishing, including electroplating and galvanizing. For example, cyanide arises in iron and steel manufacture by reduction of carbonate in the presence of carbon and nitrogen. In power plants coal burning may afford coke oven gas with a hydrogen cyanide concentration on the order of 2 grams per liter. Cyanide solutions are an important component of electroplating and galvanizing, and wash water streams resulting from post-coating treatment often contain significant quantities of cyanide. The widespread prevalence of cyanide in industrial effluents coupled with their near universal toxicity to life has made it imperative to minimize cyanide concentration in aqueous streams.
Although several methods for cyanide removal previously have been taught, we recently disclosed particularly efficient and cost-effective means for oxidizing inorganic cyanide to nitrogen and carbon dioxide using a class of metal chelates, most usually dispersed on a support. See U.S. Pat. Nos. 5,120,453 and 5,273,663. Such materials are effective for inorganic cyanide removal but they are ineffective at removing organic cyanide by means other than adsorption. In this application we disclose that when the support for the aforementioned metal chelate is one of the class of metal oxide solid solutions related to hydrotalcite, the resulting material is effective as a catalyst for both the oxidation of inorganic cyanides and the hydrolysis of organic cyanides. Not only does our material catalyze the hydrolysis of organic cyanides where the prior art catalyst fails, but our material also effects a more rapid oxidation of inorganic cyanide than does our own prior art catalyst. Both properties of the catalyst of our present invention are utterly unexpected from the teachings of the prior art.